In the production of glass panes, plates and sheets (hereinafter "panes" or "bodies"), it is frequently necessary to handle the glass body after it has been heated or during the heating thereof, e.g. in association with a heating treatment for prestressing or destressing the glass panes.
Thus, it is a common practice to provide a treatment station, e.g. a kiln, furnace or oven or a cooling chamber, through which the glass panes are displaced on a roller grate. A roller grate is a conveyor and can be an endless conveyor passing around direction-change drums, rollers or sprockets at opposite ends so as to form an upper pass and a lower pass, the endless member being provided with spaced-apart rollers or rolls which carry the glass panes on the upper pass.
In the heat treatment of glass panes, the glass is warmed to a temperature which may exceed the softening temperature so that the weight of the glass itself can cause the glass pane to sag between supported locations.
In order to prevent or reduce such sag (see German Pat. No. 704,219), means is provided to impart a back-and-forth movement to the pane-supporting rollers.
When the treatment is completed this back-and-forth movement is terminated and the non-rotated rollers move at the speed of the grate in translation to withdraw the treated glass panes from the station.
The grate may be used to displace the glass panes into the treatment station, may be immobilized while the glass panes are in the treatment station (while the rollers receive a back-and-forth angular displacement), and may carry the glass panes after treatment from the station.
With the conveyor approach, however, it is not possible simultaneously to avoid sag and carry out a continuous treatment of the glass pane, i.e. a continuous movement of the glass panes through the treating station. In other words, treatments using grates or conveyors have hitherto required an interruption of the movement of the glass panes through the treatment stage or in the displacement of the grate. Furthermore, rather complex means was required for imparting the back-and-forth angular displacement to the rollers during the phases in which the grate was immobilized. With renewed movement of the grate to carry the glass panes out of the treatment stage, the danger of sagging again presented itself, especially when the glass panes were subjected to very high temperatures during the treatment.
The treatment chamber, generally an oven, then had to be operated at lower temperatures than would be otherwise desirable.
Mention should also be made of the fact that it is known to provide apparatus for the treatment of hot glass panes, for the heat treatment of glass panes and especially prestressing devices for glass panes, in which the glass panes are displaced only by rotation of rollers driven at a predetermined angular speed which can be continuous or stepped so that the glass panes move with a speed in the transport direction, which corresponds to the product of the roller radius and the angular velocity of the glass panes. In this case as well, sagging could and usually did occur unless the transport velocity was made unusually high.
Of course, an important disadvantage of high linear transport speed for the glass panes is that the treatment stages must be made correspondingly longer for a given residence time and the extremely long units which must be used are many times more expensive and difficult to control.